1. Field of the Invention
This invention relates to a coating apparatus for magnetic recording mediums used as magnetic tapes or floppy disks. More particularly it relates to a coating apparatus that can provide two layers by simultaneous coating in a wet state and can give a smooth coating surface.
2. Description of the Prior Art
In recent years, as magnetic recording mediums are made to have higher performances, it has attracted notices to form magnetic layers in multiple layers. For example, a magnetic layer having good electromagnetic conversion characteristics in a high-density region is provided as an upper layer and a magnetic layer having better electromagnetic conversion characteristics in a low-density region than the upper layer is formed as a lower layer, so that it becomes possible to attain superior electromagnetic conversion characteristics that have not been achieved by single-layer mediums. In the case of, for example, video magnetic recording mediums of double-layer structure, the lower layer magnetic layer is so formed as to have a coating thickness larger than the upper layer magnetic layer, which is in the range of from 0.5 .mu.m to 4 .mu.m, and the upper layer is so formed as to have a coating thickness of from 0.2 .mu.m to 0.8 .mu.nm. A coating thickness larger than 0.8 .mu.m, of the upper layer causes a lowering of the characteristics at a low-frequency band, and a thickness smaller than 0.1 .mu.m, of the lower layer causes a lowering of characteristics at a high-frequency band because of a strong influence from the lower layer. It is desired for the magnetic recording mediums having such double-layer structure to be formed by coating and drying carried out once. A coating apparatus for providing two layers by simultaneous coating is disclosed, for example, in Japanese Laid-open Patent Application No. 63-88080.
Researches made by the present inventors, however, have revealed that lengthwise stripe pattern of uneven coating (stripe pattern coating faults) are produced when a magnetic coating solution for high-density recording is applied by the use of the above conventional coating apparatus. FIG. 4 shows a result obtained by measuring with a three-dimensional surface profile analyzer the surface of a coating which was formed using a conventionally known coating apparatus on a polyethylene terephthalate film of 14 .mu.m thick, coated with a magnetic coating solution composed as shown later in Table 1 to form an upper layer in a dried coating thickness of 0.3 .mu.m and also coated with a magnetic coating solution composed as shown later in Table 2 to form a lower layer in a dried coating thickness of 3 .mu.m, followed by orientation, drying and then calendering. In order to make it easy to distinguish the projected portions of the coating surface, the result of measurement shows, in the three-dimensional display, only the part higher than a mean value in the height direction. As is recognized from the observation of the drawing, stripe pattern coating faults with a pitch of about 50 .mu.m are seen on the coating surface in the direction of support travel. The mean surface roughness (hereinafter "RMS") of the coating surface was 15.8 nm. Electromagnetic conversion characteristics were also measured using an MII format deck to obtain the result that it was -2 dB on the basis of a standard tape of the applicant company and -1 dB in S/N ratio, in the video band output (7 MHz).
As is seen from the above results, the stripe pattern coating faults on the coating surface cause a serious lowering of electromagnetic conversion characteristics.
The present inventors investigated the causes of such stripe pattern coating faults to reveal that the following was responsible therefor.
In a magnetic coating solution, because of the influence of magnetic force acting between particles, it is difficult to presume that magnetic powder particles are present as primary particles. It can be presumed that they form a three-dimensional network structure and, when shear is applied thereto, they are broken into agglomerates having a certain size (Coating Engineering, Vol. 21, No. 10, pp. 475-479, 1986). In a magnetic coating solution in which a magnetic powder with a large magnetization and smaller average particle diameter is used, the particles are more strongly agglomerated. Hence, the above agglomerates are present on the order of several tens to hundreds .mu.m in the coating solution which is flowing. When this magnetic coating solution is applied to a support by means of the conventional coating apparatus, the agglomerates are forced out onto a lip from a slit of the conventional coating apparatus and thereafter the lip surface can not be well smoothed. Thus the stripe pattern coating faults are produced.
The present inventors have also confirmed that such stripe pattern coating faults become more conspicuous when a support is coated with a magnetic coating solution in which a magnetic powder with larger magnetization and smaller average particle diameter is used. At present, magnetic recording mediums trend toward high-density recording and, for that purpose, a magnetic powder with high magnetic force and ultrafine particles is used. Hence, the above stripe pattern coating faults on the coating surface give a disadvantage that is fatal in view of product quality in the sense that they cause a serious lowering of electromagnetic conversion characteristics such as video band output and S/N ratio.
It has become clear that the relationship between a second lip and a third lip must be delicately controlled before uniform and stable coating can be carried out, particularly when two layers comprising an upper layer formed to have a very small coating thickness of from 0.2 .mu.m to 0.8 .mu.m are formed by simultaneous coating. The present invention was thus accomplished.